Nuclear Magnetic Resonance (NMR) measurements of oil properties have been used to aide in reservoir fluid characterization since the early 1950s. Today, the detailed knowledge of fluid composition is a key ingredient for successful management of oilfield reservoirs. The oil composition determines the Pressure-Volume-Temperature (PVT) behavior. Viscosity measurements in addition to the T1, T2, diffusion, chain length, chemical structure, emulsion, waxing, and phase transition are all important fluid properties. Viscosity can be used as a fingerprint for other reservoir properties, such as compartmentalization. Compartmentalization is the situation in which some reservoirs have multiple levels of ‘pay zones’ and these may or may not be hydraulically connected. This has implications for the production and completions design and execution. During production there can be asphaltene drop-out or waxing problems that can cause loss of production. It is highly desirable to measure fluid properties under downhole conditions since many properties depend critically on temperature and pressure. It is also well understood that samples can undergo irreversible changes as they are extracted from the formation and transferred to the laboratory.
There are various downhole tools such as the MDT and the CHDT (trademarks of Schlumberger) tools that can be useful in obtaining and analyzing fluid samples. The downhole tools such as the MDT tool (see, e.g., U.S. Pat. No. 3,859,851 to Urbanosky, and U.S. Pat. No. 4,860,581 to Zimmerman et al., which are hereby incorporated by reference herein in their entireties) typically include a fluid entry port or tubular probe cooperatively arranged within wall-engaging packers for isolating the port or probe from the borehole fluids. It is noted they also include sample chambers which can be coupled to the fluid entry by a flow line having control valves arranged therein.
Regarding NMR measurements, the fluid sample is located in a static magnetic field and exposed to resonant frequencies (RF) pulses. The generated RF signal, produced by the irradiating RF pulses to the fluid sample, is then recorded. Depending on the exact configuration of the static field and the details of the RF pulses that are applied to the sample, many different measurements can be implemented. With uniform static fields, NMR spectra can be recorded that give information on the chemical composition of the fluids. If the static field is made non-uniform, either by applying a static field that is spatially non-uniform or by applying pulsed gradient fields, NMR imaging, diffusion and flow measurements become possible. In all cases, relaxation measurements can also be performed.